1. Field of the Invention
The present invention is directed to providing novel spectrally (optically) sensitized radiation recording photographic elements.
2. Description of the Prior Art
In accordance with techniques disclosed in the prior art, photosensitive elements and particularly photosensitive silver halide elements may be provided with increased electromagnetic radiation absorption and photochemical response by specified sensitization procedures.
Among such procedures is found a technique categorized, and denoted, as chemical sensitization, wherein a photosensitive element, and particularly a photosensitive silver halide element, may be treated with compounds such as various sulfur compounds, for example, those set forth in U.S. Pat. Nos. 1,574,944; 1,623,499 and 2,410,689; salts of noble metals such as ruthenium, rhodium, palladium, iridium and platinum, all of which belong to Group III of the Periodic Table of Elements and have an atomic weight greater than 100, for example, potassium chloroplatinate, sodium chloropalladite, ammonium chlororhodinate, and the like, in amounts below that which produces any substantial fog inhibition, as described in U.S. Pat. No. 2,488,060; gold salts, for example, potassium aurothiocyanate, potassium chloroaurate, auric trichloride, and the like, as described in U.S. Pat. Nos. 2,597,856 and 2,597,915; reducing agents such as stannous salts, for example, stannous chloride, as described in U.S. Pat. No. 2,487,850, individually or in combination. Such chemical sensitization procedures provide increased response to electromagnetic radiation by the photoresponsive silver halide treated over the frequency range of the inherent, or natural, response characteristics of the crystal.
A second procedure comprises a technique categorized, and denoted, as a spectral, or optical, sensitization procedure, wherein a photosensitive material, and particularly photosensitive silver halide, is provided frequency-selective electromagnetic radiation response characteristics and/or an increase in its inherent, or natural, spectral response characteristics.
In general, such spectral sensitization procedures are accomplished by the adsorption onto one or more surfaces of the photosensitive material of one or more dyes selected from certain classes of dyes including, preferably, cyanine dyes and dyes related to them. For an extensive treatment of cyanine dyes particularly adapted to provide spectral sensitization of, for example, a photosensitive silver halide crystal see Hamer, F. M., The Cyanine Dyes and Related Compounds, Interscience Publishers, New York, N.Y., U.S.A., (1964).
By means of the traditional procedures disclosed in the art as adapted to accomplish spectral sensitization of photosensitive material, and preferably sensitization of photosensitive silver halide, a cyanine dye in the form of polymeric aggregates is adsorbed to the receptive faces, or surfaces, of the photoresponsive material in a statistical monomolecular layer thickness or less. Generally, the cyanine dyes preferably employed for purposes of spectral sensitization comprise an amidinium ion system in which both of the nitrogen atoms are included within separate heterocyclic ring systems, and in which the conjugated chain joining the nitrogen atoms passes through a portion of each heterocyclic ring merocyanine. Adsorption is generally believed to be partly accomplished by an unknown type of chemiadsorption between negative crystal surface charges provided, for example, by the excess halide components of the silver halide, and the positive charge of the cyanine chromophore. Adsorption is also favored by the ability to form silver complexes with nuclei containing an amidinium nitrogen atom of a selected cyanine dye's heterocyclic ring system, or systems, for example, with a nuclear sulfur, oxygen, or selenium atom, or a second nuclear nitrogen atom not directly a component of the amidinium ion system.
It has also been understood that the efficiency of the spectral sensitization of a, for example, silver halide crystal increases in accordance with an increase in the chemiadsorption of the selected sensitizing dye, in the form of polymeric aggregates, on the appropriate surfaces, or faces, of the crystal up to the concentration at which increase of sensitivity peaks or plateaus. Specifically, maximum sensitization has been found ordinarily to occur at a dye concentration level less than or equal to a statistical monomolecular layer of dye coverage on the adsorbing surfaces of the crystal, usually just short of monomolecular coverage of the crystal surface.
Sensitivity conferred by a sensitizing dye thus does not increase proportionately to the concentration of the dye, but rather passes through a maximum as concentration is increased. Attempts to increase the spectral sensitivity of the crystal by increasing the concentration of sensitizing dye adsorbed by its appropriate surfaces beyond the plateau or peak concentration level, provide a progressive decrease in spectral sensitivity as the concentration is so increased; see: Hamer, F. M., The Cyanine Dyes and Related Compounds, supra, and Borin, A. V., Investigation of the Concentration Effect in Optical Sensitization of Photographic Emulsions, Uspekhi Nauch. Fab. Akad. Nauk. SSSR, Otdel. Ihim. Nauk. 7, 183-190 (1960). In many instances, this resultant decrease in the crystal's spectral sensitivity attains catastrophic proportions when the relative amount of dye necessary to provide a given incremental increase in sensitivity, prior to attainment of the plateau or peak region, is compared with the same amount of dye, in excess of that which provides optimum sensitization.
The energy or charge-carrier absorptive propensity of a photoresponsive element comprising a particulate dispersion of photosensitive material is generally dependent upon the effective, adsorbed presence of sufficient dye to effect maximum absorption of, and transfer of, electromagnetic energy-induced photoreaction stimulus to the photosensitive material. The aforementioned monomolecular layer adsorption of the dye to the appropriate surfaces of the photosensitive material fails, by a relatively large degree, to provide complete absorption of incident radiation. In fact, in conventional optically sensitized, photographic, photoresponsive elements, such as panchromatic photographic emulsions, coated on a suitable supporting member, comprising a relatively thin layer, for example, on the order of about 7 microns in thickness, and including a dispersion of photoresponsive silver halide in a gelatin matrix, for example, in a concentration of about 100 mgs. of silver per square foot, the photoresponsive element only absorbs roughly in the order of less than one-third of the available incident light, over the radiation frequency range desired for photographic employment of the element, with the concomitant failure of such elements to even approximate their potential, or theoretical, efficiency. The maximum absorbed radiation attributable to a given monomolecular dye layer adsorbed on a photosensitive crystal is about 7 percent of the total incident radiation. W. West and V. I. Saunders, Wissenschaftliche Photographie, W. Eichler, H. Frieser and O. Helwich, eds., Verlag Dr. O. Helwich, Darmstadt, 1958, p. 48. The net response of the system thus cannot be improved by simply adding more of the same sensitizing dye, but must be achieved by development of more efficient and effective sensitizing dye systems.
In U.S. Pat. Nos. 2,213,995 and 2,231,658 there is disclosed cyanine dyes wherein X.sup.+ of Formula I, infra, is a --COOH group and Y.sup.- is a --COO.sup.- group; U.S. Pat. Nos. 2,503,776; 2,519,001; and 2,912,329, and British Pat. No. 742,112, German Pat. No. 929,080, Belgian Pat. No. 532,409 disclose cyanine dyes wherein X.sup.+ of Formula I is a --SO.sub.3 H group and Y.sup.- is a --SO.sub. 3 .sup.- group; German Pat. No. 1,028,718 discloses cyanine dyes wherein X.sup.+ of Formula I is an alkyl group and Y.sup.- is a --O--SO.sub.3 .sup.- group; German Pat. No. 1,916,845 discloses cyanine dyes wherein X.sup.+ of Formula I is a --S--SO.sub.3 H group; and Y.sup.- is a --S--SO.sub.3 .sup.- group; and U.S. Pat. Nos. 2,256,163 and 2,354,524 disclose cyanine dyes wherein X.sup.+ of Formula I is a --.sup.+N(alkyl).sub.3 group and Y.sup.- is an alkyl group.